Implantable medical devices are used to treat patients suffering from a variety of conditions. Examples of implantable medical devices include implantable pacemakers and implantable cardioverter-defibrillators (ICDs), which are electronic medical devices that monitor the electrical activity of the heart and provide electrical stimulation to one or more of the heart chambers as necessary. Pacemakers deliver relatively low-voltage pacing pulses in one or more heart chambers. ICDs can deliver high-voltage cardioversion and defibrillation shocks in addition to low-voltage pacing pulses
Pacemakers and ICDs generally include pulse generating circuitry required for delivering pacing and/or cardioversion and defibrillation pulses, control circuitry, telemetry circuitry, and other circuitry that require an energy source, e.g. at least one battery. In addition to a battery, ICDs include at least one high-voltage capacitor for use in generating high-voltage cardioversion and defibrillation pulses. Implantable medical devices (IMDs), including pacemakers, ICDs, drug pumps, neurostimulators, physiological monitors such as hemodynamic monitors or ECG monitors, typically require at least one battery to power the various components and circuitry used for performing the device functions.
IMDs are preferably designed with a minimal size and mass to minimize patient discomfort and prevent tissue erosion at the implant site. Batteries and capacitors, referred to collectively herein as “electrochemical cells,” contribute substantially to the overall size and mass of an IMD. Electrochemical cells used in IMDs are provided with a hermetically-sealed encasement for housing an electrode assembly, including an anode and cathode separated by a separator material, an electrolyte, and other components such as electrode connector feed-throughs and lead wires. The encasement includes a case and a cover that are sealed after assembling the cell components within the case.
The total amount of the anode and cathode material required in the cell will depend on the energy density, volume, voltage, current, energy output, and other requirements of the cell for a particular application. Anode and cathode material, with an intervening separator, may be arranged in a coiled electrode assembly. Both round and flat cylindrical coiled electrode assemblies are known in the art. Flat electrochemical cell designs tend to improve the volumetric efficiency of the cell because they are generally better suited for fitting within an IMD housing with other device components. Flat electrochemical cell designs may utilize a stacked electrode assembly wherein anode, cathode and intervening separator material are arranged in a stacked configuration.
The implementation and use of high voltage output systems within ICDs is well known. Generally, ICDs have high voltage (HV) output capacitors, typically valve metal electrolytic capacitors, which are typically charged to a substantially full (or maximum) preprogrammed charge via high current battery systems, such as lithium/silver vanadium oxide (SVO) battery cells, coupled to DC-to-DC voltage converters in order to generate cardioversion/defibrillation (C/D) shocks. The HV output capacitors are charged up to the programmed voltage when tachyarrhythmia detection criteria are met and a C/D shock is to be delivered by discharging the HV output capacitors through the heart between C/D electrodes.
The term “valve metal” stands for a group of metals including aluminum, tantalum, niobium, titanium, zirconium, etc., all of which form adherent, electrically insulating, metal oxide dielectric films or layers upon anodic polarization in electrically conductive solutions. Valve metal electrolytic capacitors have a relatively high energy density per unit volume making them volumetrically efficient in terms of the energy available.
The performance of valve metal and other types of capacitors depends upon several factors (e.g., the effective surface area of the anodes and cathodes that can be contacted by electrolyte, the dielectric constant of the oxide formed on the anode surface, the thickness of the dielectric layer, the conductivity of the electrolyte, etc.). The thickness of the dielectric layer is determined by the anodization method used and the anode substrate material.
Wet electrolytic capacitors essentially consist of an anode electrode, a cathode electrode, a barrier or separator layer for separating the anode and cathode, and a liquid electrolyte. In cylindrical electrolytic capacitors, the anode electrode is typically composed of wound anodized aluminum foil in which subsequent windings are separated by at least one separator layer. The anodes in a flat electrolytic capacitor (FEC) may consist of stacked sheets of aluminum that are electrically connected together. In a slug or pellet type of capacitor a valve metal powder is pressed, sintered and formed into a typically unitary anode electrode, and the anode is separated from at least one cathode by a electrically insulative separator layer as is known in the art and as described further below. For an FEC, typically a plurality of aluminum sheets are etched or perforated to increase surface area. For both FEC- and pressed and sintered-type capacitors, an oxide dielectric is formed upon exposed surfaces of the anode (the pressed and sintered structure or etched or the perforated sheets) when the anode is immersed in a formation electrolyte while electrical current circulates therethrough during manufacture. Examples of electrolytic capacitors are disclosed, for example in commonly assigned U.S. Pat. No. 6,006,133.
As it is desirable to minimize overall IMD size and mass, electrochemical cell designs that allow cell size and mass to be reduced are desirable. Reduction of capacitor cell size and/or mass, without reducing the available energy, may allow balanced addition of volume to other IMD components, thereby increasing device longevity and/or increasing device functionality.